During the last few decades, an increasing number of commercial vehicles used for field work in many industries have been equipped with auxiliary hydraulic power supplies that include one or more fixed displacement hydraulic pumps driven by the vehicle's primary engine. In almost all instances, the vehicle's engine is an internal combustion (I.C.) engine having a conventional throttle arrangement for controlling the speed of the engine via driver-applied pressure upon an accelerator pedal connected by a linkage to the engine's throttle. The throttle and linkage are biased by one or more mechanical springs to return to their idle position. The engine-driven pumps are often used to power manually controlled, hydraulically driven power tools, as well as other hydraulic accessories or attachments.
Such power tools may be categorized into sizes by their hydraulic power requirements, which can be conveniently expressed in terms of flow rate and pressure. The larger tools, such as jack hammers and tampers, may, for example, require a flow rate of about 7 to 9 gallons per minute (gpm) at a pressure of roughly 1300 to 1700 pounds per square inch (psi) for optimal operation. Smaller tools, which include pruners, saws and concrete drills, may, for example, optimally require a flow rate of 3 gpm at a pressure of roughly 1500 to 1900 psi.
In the past, a number of different hydraulic throttle control arrangements have been used to regulate engine speed to maintain a selected output parameter of a hydraulic power supply, such as a flow rate, in a relatively constant state when the power tool attached thereto is operating. One such arrangement employs a hydraulically operated throttle control device, known as the Hunter Hydro-Throttle Control Model No. STA9010, which is available from Muncie Parts Mfg. Co., Inc., Muncie, Indiana. This device, which is customarily rigidly mounted to the engine block or a bracket extending therefrom, includes a cable that is attached to the throttle linkage of the engine. When the hydraulic power supply is not in use, the cable is slack, and thus has no effect on the normal operation of the engine by the vehicle's driver. When the hydraulic power supply is to be used, the vehicle is kept stationary and the engine left on, which allows a throttle control device to establish the desired engine rpm (revolutions per minute) at the appropriate time. With the Hunter device, this occurs when the device senses that a tool connected to the hydraulic pump outlet requires power. The device then moves the attached cable a predetermined distance, thus pulling the throttle to a predetermined position to produce the engine rpm necessary to achieve the desired pump flow rate for the tool.
The Hunter device, once properly installed and adjusted, operates satisfactorily in those situations where only one flow rate is required from the pump. Problems have been encountered, however, when using this device or similar devices in situations where both large and small hydraulic tools are driven from the same pump. Unless these devices are adjusted to match the flow requirements of the particular tool connected to the pump, there will be either too much or too little flow to at least one of the tools from the pump. The problems associated with too little flow are insufficient power to run a large tool, low tool speed and stalling. To remedy these problems, the devices are set to operate the engine so as to produce a flow rate adequate for the large tools used with the pump. This in turn creates several problems for small tools, such as excessive hydraulic pressure, overspeeding and accelerated tool wear. It also causes unnecessary fuel consumption, engine wear, noise and air pollution. More importantly, it often causes the entire auxiliary hydraulic system to overheat, which leads to many problems and substantially increases maintenance costs. The overheating problem is primarily due to the excess flow from the pump dumping over the system relief valve, which is typically set at 2000 psi or above.
It is the primary object of the present invention to provide an improved hydraulic throttle control apparatus which remedies the foregoing problems.
Another object of the present invention is to provide an improved hydraulic throttle control apparatus that senses the hydraulic needs of a power tool attached to an engine-driven fixed displacement hydraulic pump, and responds thereto by selecting one of two predetermined engine speeds for driving the pump.
Still another object is to provide an improved hydraulic throttle control apparatus for regulating the flow rate of an engine-driven fixed displacement pump which can respond to the hydraulic needs of two different power tools simultaneously connected to the apparatus.
A further object is to provide an improved hydraulic control apparatus for the foregoing objectives that is relatively inexpensive, simple to install and adjust, and relatively compact so as to fit under the hood of the vehicle's engine compartment.
Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.